#include <iostream>
#include "Sales_item.h"

int main()
{
	Sales_item item1, item2;
	std::cin >> item1 >> item2;

	if (item1.isbn() == item2.isbn()) {
		std::cout << item1 + item2 << std::endl;

		return 0;
	} else {
		std::cerr << "Data must refer to same ISBN" << std::endl;

		return -1;
	}

	return 0;
}
